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## Terminator (solar) A terminator or twilight zone is a moving line that separates the illuminated day side and the dark night side of a planetary body. A terminator is defined as the locus of points on a moon or planet where the line through its sun is tangent.

At the Equator, under flat conditions (without obstructions like mountains or at a height above any such obstructions), the terminator moves at approximately 1,668 kilometers per hour (1,036 miles per hour). This speed can appear to increase when near obstructions, such as the height of a mountain, as the shadow of the obstruction will be cast over the ground in advance of the terminator along a flat landscape. The speed of the terminator decreases as it approaches the poles, where it can reach a speed of zero (full-day sunlight or darkness).


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